1. Field of the Invention
The present invention relates to a single crystal silicon substrate which contains inorganic impurity atoms and which substrate is used as a basic material for obtaining a silicon film by a hetero-epitaxial growth method, or manufacturing a high quality semiconductor device.
2. Description of the Related Art
A high quality semiconductor device comprises a single crystal silicon substrate made from an insulation material such as a glass plate on which a single crystal silicon film is formed. The substrate is produced by a zone melt recrystallization method.
However, the method involves a problem of grain boundary. To cope with the problem, a method has been adopted in which the grain boundary is generated only in a region where the material thereof does not influence the characteristic of the substrate so as to avoid the degradation of the device due to the grain boundary. More precisely, a film layer of Si.sub.3 N.sub.4 is disposed in a form of strips on a surface of the substrate covered with a film of SiO.sub.2. The Si.sub.3 N.sub.4 film absorbs optical rays so that the region under the film is heated. As a result, recrystallization of the heated region is retarded so that grain boundaries of different crystal face are concentrated at the region. Therefore, by disposing the Si.sub.3 N.sub.4 film at a desirable region, it becomes possible to generate the grain boundaries only in that region of the substrate.
However, in accordance with the grain boundary control method mentioned above, heat distribution is changed according to a slight difference of film quality and film thickness so that the refractive index of the substrate is changed due to a slight change of the film quality as well as the optical absorption coefficient thereof, which makes it difficult to obtain a substrate of stable and even characteristic distribution. Besides, cracks are generated due to the difference of film thickness and film quality in the substrate.